Not applicable.
Not applicable.
1. Field of the Invention
The invention relates generally to the field of electromagnetic well logging instruments. More specifically, the invention relates to methods and apparatus for electromagnetic resistivity logging which can measure properties of formations not yet penetrated by a wellbore, or axially xe2x80x9cahead of the bit.xe2x80x9d
2. Background Art
There are two general classes of electromagnetic (EM) resistivity well logging instruments known in the art. One such class is known as electromagnetic propagation resistivity logging, the other is known as electromagnetic induction resistivity logging.
EM propagation resistivity logging of earth formations is typically performed by an instrument that forms part of a drill string. Such instruments are commonly referred to as xe2x80x9clogging while drillingxe2x80x9d (LWD) instruments. Typical EM propagation LWD instruments include one or more transmitter antennas, and a plurality of receiver antennas disposed on a drill collar. Radio frequency (RF) power, usually at a frequency in a range of 0.1 to 10 MHz is passed through the one or more transmitter antennas, and an amplitude and phase of RF voltages induced in the receiver antennas are measured. Generally, the conductivity of earth formations proximate any pair of receiver antennas is related to the amplitude ratio and phase difference of the induced voltages between the receiver antennas.
In most EM propagation resistivity LWD instruments, the antennas are formed as loops or coils wound around the exterior of the instrument or drill collar so that they form magnetic dipoles having moments substantially parallel to the axis of the instrument. Such a configuration makes the instrument mainly responsive to the formations disposed laterally around the wellbore proximate the antennas. Various combinations of transmitter and receiver antennas are known in the art which provide responses in particular selected axial positions along the drill collar, and at selected lateral depths from the wellbore, but the sensitivity of most EM propagation LWD instruments is primarily laterally around the instrument. Such sensitivity is most useful when the instrument axis is substantially perpendicular to the attitude (xe2x80x9cdipxe2x80x9d) of the various layers of the earth formations penetrated by the wellbore. This is typically the case where the dip is close to zero or equal to zero and the wellbore is drilled substantially vertically. Many wellbores, however, are drilled at substantial inclination, up to and including horizontal, and many earth formations have substantial dip inclination from horizontal. In such cases, or any combination thereof which results in high xe2x80x9capparent dipxe2x80x9d (angle between wellbore inclination and formation dip), lateral sensitivity is less useful. An LWD resistivity instrument which has substantial axial sensitivity would be desirable in these cases, in one particular application to detect the presence of resistivity discontinuities which have not yet been penetrated by the wellbore (axially xe2x80x9cahead of the bitxe2x80x9d).
An important advantage offered by LWD instruments is that measurements of properties of earth formations penetrated by a wellbore can be transmitted to the earth""s surface substantially in xe2x80x9creal timexe2x80x9d, meaning while such formations are actually being penetrated by the drilling thereof, or very shortly thereafter. Such capability can provide the wellbore operator with information that may improve drilling efficiency, improve the accuracy of characterization of fluid content of the formations, and reduce risk of unexpectedly encountering drilling hazards. It is particularly desirable, therefore, to have an electromagnetic propagation LWD instrument which has axial sensitivity in formations xe2x80x9cahead of the bitxe2x80x9d, meaning formations which have not yet been penetrated by the wellbore while the well is being drilled.
EM induction logging instruments include a source of alternating current which is coupled to a transmitter antenna. The alternating current has a frequency typically in a range of 10 to 200 KHz. The alternating current passing through the transmitter antenna induces time varying EM fields in the earth formations surrounding the instrument. One or more receiver antennas are disposed on the instrument at selected positions. The receivers are coupled to circuits in the instrument which are adapted to determine components of the voltages induced in the receiver antennas which have magnitudes related to the magnitude of eddy currents induced in the formations by the time varying EM fields (the latter being ultimately induced by the transmitter current). As is well known in the art, the magnitude of the eddy currents, and the corresponding receiver voltage components, are related to the electrical conductivity of the earth formations. Typically, the transmitter and receiver antennas are arranged on induction logging instruments so that the response of the instrument is primarily related to conductivity of the formations disposed laterally about the instrument, in a manner similar to propagation logging. More specifically, the region about the instrument to which the measurements mainly correspond is generally disposed axially between the transmitter and receiver antennas. Various arrangements of transmitter and receiver antennas are known in the art to further delimit the sensitive region to various axial and lateral (radial) zones about the instrument, but generally speaking, the response of the instrument is mostly confined between the axially endmost transmitter and receiver antennas on the instrument. As is the case for LWD instruments, it is desirable to have an induction logging instrument which can measure in a zone as yet to be penetrated by a wellbore.
One aspect of the invention is an electromagnetic well logging instrument which includes at least one transmitter antenna oriented to have at least some magnetic dipole moment component transverse to an axis of the instrument. The instrument includes at least two receiver antennas oriented to have at least some magnetic dipole moment component transverse to the axis and parallel to the magnetic dipole moment component of the at least one transmitter antenna. The at least two receiver antennas are spaced apart from the transmitter antenna so that a response of the instrument to the transverse magnetic dipole moment component is axially substantially on one side of a more distant spaced one of the at least two receiver antennas. In one embodiment, the instrument includes circuits coupled to the antennas adapted to make electromagnetic induction measurements of earth formations. In another embodiment, the instrument includes circuits coupled to the antennas adapted to make electromagnetic propagation measurements of the earth formations. In a particular embodiment, a ratio of spacing from the transmitter of the closer one of the receiver antennas to the spacing from the transmitter of the more distant one of the receiver antennas is about 0.563.
Another aspect of the invention is an electromagnetic well logging instrument which includes at least one receiver antenna oriented to have at least some magnetic dipole moment component transverse to an axis of the instrument. The instrument further includes at least two transmitter antennas oriented to have at least some magnetic dipole moment component transverse to the axis and parallel to the magnetic dipole moment component of the at least one receiver antenna. The at least two transmitter antennas are axially spaced apart from the receiver antenna so that a response of the instrument to the transverse magnetic dipole moment component is axially substantially on one side of a more distant spaced one of the at least two transmitter antennas. In another embodiment, the instrument includes circuits coupled to the antennas adapted to make electromagnetic propagation measurements of the earth formations. In a particular embodiment, a ratio of spacing from the receiver antenna of the closer one of the transmitter antennas to the spacing from the receiver antenna of the more distant one of the transmitter antennas is about 0.563.
A method for electromagnetic resistivity well logging according to another aspect of the invention includes passing alternating current through a transmitter antenna disposed in a wellbore. The transmitter has at least some dipole moment component in a direction perpendicular to an axis of the wellbore. The method then includes detecting voltages induced in at least two receiver antennas each having at least some dipole moment component parallel to the dipole moment component of the transmitter antenna. The at least two receiver antennas are axially spaced apart from the transmitter antenna such that a response of the receivers is substantially axially to one side of the more distant one of the receiver antennas.
A method for electromagnetic resistivity well logging according to another aspect of the invention includes passing alternating current through at least two transmitter antennas disposed in a wellbore. The transmitter antennas have at least some dipole moment component in a direction perpendicular to an axis of the wellbore. The method then includes detecting voltages induced in a receiver antenna having at least some dipole moment component parallel to the dipole moment component of the transmitter antennas. The at least two transmitter antennas are axially spaced apart from the receiver antenna such that a response of the receiver antenna is substantially axially to one side of the more distant one of the transmitter antennas.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.